The anatomy of a cephalochordate is diagrammed at left. Note that cephalochordates have all the typical chordate features. The dorsal nerve cord is supported by a muscularized rod, or notochord. The pharynx is perforated by over 100 pharyngeal slits or "gill slits", which are used to strain food particles out of the water. The musculature of the body is divided up into V-shaped blocks, or myomeres, and there is a post-anal tail. All of these features are shared with vertebrates. On the other hand, cephalochordates lack features found in most or all true vertebrates: the brain is very small and poorly developed, sense organs are also poorly developed, and there are no true vertebrae.
Water is taken in through the mouth, drawn in by the beating of cilia located on the wheel organ, a set of ridges lying inside the mouth. The water is first filtered by the oral cirri, slender projections that surround the opening of the mouth, clearly visible on the photograph at the top of the page. It then passes through the gill slits. These gill slits are enclosed by folds of the body wall, the metapleural folds, to form a body cavity known as the atrium. Food particles in the water are trapped by mucus, while water passes through the slits and out of the atrium through the atriopore, located towards the posterior end. The rest of the digestive system is fairly simple: a pouch or hepatic caecum secretes digestive enzymes, and actual digestion takes place in a specialized part of the intestine known as the iliocolonic ring. Cephalochordates also have a well-developed circulatory system and a simple excretory system composed of paired nephridia. The sexes are separate, and both males and females have multiple paired gonads. Eggs are fertilized externally, and develop into free-swimming, fishlike larvae.
Since cephalochordates have no hard parts, their fossil record is extremely sparse. However, fossil cephalochordates have been found in very old rocks indeed, predating the origin of the vertebrates. The famous Middle Cambrian Burgess Shale of British Columbia has yielded a few fossils of Pikaia, which appears to be a cephalochordate (although the fossils are still being restudied). More recently, Yunnanozoon, from the Early Cambrian of south China, was reported to be a cephalochordate, the earliest known (Chen et al., 1995). These fossils show that the chordate lineage appeared very early in the known history of the animal kingdom, and they strengthen the case for an origin of true vertebrates from a cephalochordate-like ancestor.
Today, amphioxus may be extremely common in shallow sandy environments: at Discovery Bay, Jamaica, up to five thousand individuals per square meter of sand have been reported. In some parts of the world, amphioxus are eaten by humans or by domestic animals; they are important food items in some parts of Asia, where they are commercially harvested.
Chen, J.-Y., Dzik, J., Edgecombe, G.D., Ramsköld, L., and Zhou, G.-Q. 1995. A possible Early Cambrian chordate. Nature 377: 720-722.
Ruppert, E.E. and Barnes, R.D. 1994. Invertebrate Zoology. Sixth edition. Saunders College Publishing, Fort Worth.